Devices have been proposed to convert an electrical signal, representative of image information, to a corresponding imagewise pattern of light. Such conversion devices, hereinafter "image sources", can provide a light image or portion thereof for image display, imagewise exposure of a photosensitive medium, or for delivery to an image information transmission or sensing system.
For example, in a typical image recording system, an image source may be used to provide a pattern of light that exposes and thus alters a photosensitive recording medium in an imagewise fashion. The desired image may be composed and made visible on the recording medium, such as photographic film, or upon other types of receiving media from which the image is transferred to a hard copy medium, such as those used in the electrophotographic art.
If used as an image display device, an image source is expected to produce a light pattern that is directly viewable by a human observer. The pattern may be composed by accumulating line-by-line portions of the image to be displayed, or an entire image frame may be produced substantially at once. For example, planar image display devices have found application as alpha-numeric displays.
Electro-optical Element Arrays
Some conventional image sources are constructed from arrays of minute electro-optical elements, such as liquid crystal light shutters, each of which have a limited output area of predetermined geometry and arrangement that selectively transmit or attenuate light from a diffuse light source. Most other image sources have been constructed from arrays of electro-optical point-source elements such as light-emitting diodes that directly generate light.
The foregoing arrays are typically arranged linearly, known as a strip array, or in rows and columns, to form an area array. Each image source array element is addressable according to its position in a row (in a strip array) or a row and column matrix (in an area array). Each element is activated by an addressing scheme for activating at least one address line for each point in the array. Electro-optical elements that employ active binary elements also must be individually switched between "ON" to "OFF" states in accordance with appropriate electrical input signals that are applied directly to the individual elements.
In a linear array, there is therefore at least one signal electrode for each element. In an area array, at a minimum there are "X-Y" addressing signals used to power electrode strips that control the transmission of respective row and column signals to correspondingly-addressed elements. By application of suitable potentials to two or more sets of electrodes, individual elements of the display can be switched on or off in a line-by-line manner to produce a complete image.
Such addressing requirements limit the geometry and density of such arrays, and the type and amount of image information that they can produce. The conventional image source therefore must utilize extremely large numbers of signal electrodes, and complex addressing and control methods and circuitry, if they are to meet the stringent requirements in modern, high-density, high resolution imaging applications.
Another disadvantage of conventional image sources is that the aforementioned arrays, whether they be of the linear or area array type, are difficult and costly to manufacture if they are to include a sufficient density of light source elements for the production of a high resolution image. In practice, their light output is typically limited in resolution to approximately 400-600 dots per inch (DPI).
A third disadvantage is the difficulty in achieving accurate, high-speed activation and control of the many individual elements that are selected to comprise an image pattern. This is especially difficult for processing image information that is dynamic, such as that found in video imaging systems. For example, in an area array, a row and column addressing system can be consistent with the addressing techniques used in solid state random access memories and requires little support circuitry to operate one emission or transmission point at a time. However, there is an inherent disadvantage in that such an addressing scheme becomes quite limited when the image pattern to be produced becomes more complex, more detailed, or more transitory.
For example, light emitting diodes (LED's), each of such minute size and arranged in an array of sufficient density so as to provide a high resolution image, are costly to produce as a linear array and are prohibitively expensive if organized as an area array. The interconnection and addressing requirements for an area LED array, not to mention the support circuitry, is also expected to be extremely complex and expensive.
Another inherent disadvantage is that each point in a conventional image source array may be called upon to activate for only a brief moment. The instantaneous brightness of any element must be achieved quickly and to a very high level--a requirement that has not been met in the present state of the art.
Further, conventional image sources are constructed from elements that have relatively large dimensions; these discrete elements are inherently not amenable to high packing densities, and are more prone to manufacturing defects or failure when such densities increase. Only one element is typically dedicated to producing one picture element ("pixel") of the image. Hence, upon the failure of any one image source element, such as an LED, a full "pixel" of image information is permanently lost. The overall image quality of the array is quickly degraded as more and more elements fail.
Conventional image sources that use active electro-optical elements are also relatively inefficient in converting an impressed signal to light, and therefore require large surges of driving current for their activation. Such drive currents require the use of specially-designed power supplies. A large proportion of the current in such power surges is dissipated as heat, which is undesireable and must be removed by the use of heat sinks.
Additionally, the driving signal to conventional active devices such as LED's is typically modulated to circumvent their inherent nonlinearity in light output. The typical electro-optical element is operated according to, for example, pulse-mode modulation. Activation signals for binary elements are abrupt and require a very fast, complex switching matrix or a multiplexing apparatus. The activation signal, pulsed by a current source circuit at a very high rate usually creates significantly undesireable electromagnetic interference (EMI).
Still another disadvantage of an image source based on electro-optical elements is that the array cannot generate true primary colors. LED's that emit in the blue region of the spectrum have only recently been developed and are not practically suited for most applications.
Cathodoluminescent Image Sources
Cathodoluminescent devices are also known for use as image sources in image display and image recording applications. For example, in U.S. Pat. No. 4,803,565, a conventional electrostatic recording apparatus is disclosed as having an optical write head having light emission elements disposed in rows, for forming an electrostatic latent image on a photosensitive material. The construction of the write head is said to include anode patterns formed within an evacuated, closed case. The anode patterns are coated with fluorescing material. A thermionic cathode is heated by an electric current flowing therein, causing the emission of thermal electrons. When the cathode is grounded and a positive voltage is impressed on the anode electrodes, the thermal electrons collide with the florescing material, causing it to fluoresce. To achieve a selected pattern of florescence, the anode electrodes must be distributed in spaced, parallel insulated intervals in an alternating relationship. A circuit must be provided for selectively impressing a predetermined positive voltage individually to each anode electrode to provide a predetermined pattern of light emission corresponding to the desired image dot pattern in an electrostatic latent image on the photosensitive material.
Hence, activation signals for each anode electrode element must be conducted in a complex switching matrix and may require multiplexing. Such apparatus is costly, difficult to manufacture and operate, and generates undesireable electromagnetic interference (EMI). Thermionic ("hot") cathode designs are also very inefficient, and suffer from structural degradation, device failure modes, and many other effects of the heat dissipated at the cathodes.
Much larger scale cathodoluminescent image displays, such as the cathode ray tube (CRT), are known to include a cathodoluminescent layer at the face of the screen which is written by a movable electron beam emitted from a thermionic cathode gun. Unlike the aforementioned arrays of electro-optical elements, a CRT will rather easily display image information in the form of a rapidly time-varying electrical signal in a rasterized format. That is, individual electrodes for each picture element are not used and matrixing is unnecessary. Instead, the controlled deflection of the electron beam allows the CRT to convert signals representative of a rapidly changing image to a displayed image having at fairly high light intensity. The cathode ray tube (CRT) has found widespread application mainly as a low-resolution image display system.
However, large scale cathodoluminescent image sources suffer from many intractable disadvantages. No large-scale cathodoluminescent device has been successfully designed or fabricated as a source of very high-resolution light images. Their thermionic electron source design substantially limits the efficiency of the device as well as its operating life. A CRT, for example, requires a high voltage source on the order of 10,000 to 30,000 volts. Also, these devices require a large, bulky glass envelope so as to contain the electron beam in a high vacuum environment.
There has been significant interest and much effort expended in developing satisfactory "flat panel displays" which obviate the depth requirement of a typical large-scale cathodoluminescent device, while having comparable or better light emission characteristics, e.g., brightness (efficiency), resolution, power requirements, etc. Some flat panel image sources have been produced which utilize single, multiple or ribbon beams directed initially essentially parallel to the plane of the display and then caused to change directions essentially in the Z direction to address appropriate areas of the display target either directly or by way of a selecting and/or focusing grid structure. Examples are the Aiken and Gabor devices, U.S. Pat. Nos. 2,928,014 and 2,795,729, respectively, using single guns, and the RCA multibeam channel guide system as exemplified by U.S. Pat. Nos. 4,103,204 and 4,103,205.
The major drawbacks of flat panel CRT systems still reside in their construction and/or their complex electrical and electron/optical control requirements, their dependency on a thermionic cathode structure, and the requisite ancillary apparatus for providing the high voltage fields necessary to precise electron beam steering. Known flat panel displays are useful for some minor display applications but have not been produced in panels that offer image resolution and image quality in excess of a conventional CRT. Designers of image display and recording apparatus therefore have been limited to the above-described arrays of electro-optical devices as previously described.
Field Emitter Arrays (FEA)
In search of a replacement for the conventional thermionic cathodoluminescent image source, researchers in the field of vacuum microelectronics have pursued the development of microfabricated cold electron sources capable of providing vacuum current densities that are orders of magnitude above those provided by thermionic electron sources. Vacuum microelectronics combines the arts of semiconductor solid-state processing and fabrication techniques with vacuum ballistic electron transport. Arrays of micron-scale, high-current-density cold emitters have been proposed to obviate inefficient and cumbersome low-current-density thermionic cathode designs.
Recent progress in the microfabrication of a low-voltage integrally-gated vacuum electron emitter has resulted in increasing interest in what is known as field emission cathodes (FEC), a plurality of which are constructed and arranged as a field emitter array (FEA). Conventional FEA's are arrays of tightly-packed, gated vacuum field emission devices wherein the field emission is based on the quantum mechanical tunneling of electrons through the emitter/vacuum interface upon the application of a high electric field. With field strengths of 5.times.10.sup.7 V/cm, extremely high current densities can be extracted: up to 10.sup.6 A/cm.sup.2 in metals. One advantage of field emission is that little energy is expended in extracting the electrons through the surface.
The simplest field emitter array consists of a plurality of extremely sharp micron-scale vacuum electron field emitter tips, each having an integrated conducting extraction gate on an associated dielectric layer. The electron emission is controlled by varying the gate-to-emitter voltage. Electrons emitted from the tip travel ballistically in the vacuum to a drain.
Small-scale cathodoluminescent image display devices have been proposed that comprise baseplates covered with field emitter arrays, each of which is studded with many conic, submicron-sized emitters. When the space between the backplate and a phosphor-coated faceplate is evacuated, and an appropriate voltage is established between the plates, ballistic electrons emitted from the cathodes travel in a relatively straight line to activate the phosphor dots that comprise the pixels on the faceplate. Each pixel thus has a dedicated array of cathodes.
However, other problems have prevented such a structure from being a practical image source. The cathode emitter tips are nonuniform in their output and typically are difficult to manufacture in inexpensive arrays for uniform image generation over a large areas. Furthermore, there is the long-standing problem, common to the aforementioned electro-optical element arrays, in assigning and/or switching discrete portions of an applied electrical signal to the many respective points in the field emitter array. The requisite multiplexing and/or addressing schemes, as described hereinabove, are undesireably complex and expensive when used to selectively activate the field emitter arrays. Moreover, such matrixing schemes can require the switching or modulation of fairly high voltage potentials. The task of switching such potentials at a high rate is quite difficult in practice.
Further background on field emission structures may be found in U.S. Pat. Nos. 3,789,471; 3,812,559; 3,453,478; and 4,857,799; and in Vacuum Microelectronics 1989: Second International Conference on Vacuum Microelectronics, Turner (ed.), 1989.
Solid State Displays
In a different approach, disclosed in U.S. Pat. No. 3,792,465 and entitled Charge Transfer Solid State Display, a solid state display incorporates a semiconductor charge shift register. Information for display is read into the semiconductor charge devices by shift register action in the form of minority carriers. In one embodiment, the substrate comprises a unitary body of semiconductor material having light-emitting characteristics. Means are provided for reverse biasing the p-n junction to near avalanche breakdown such that the minority carriers corresponding to the data to be displayed trigger avalanche and provide a large quantity of minority carriers for producing a visible display upon recombination with majority carriers.
Nonetheless, there is a major problem associated with a semiconductor charge transfer device that precludes its practical use as a light emitting device. Silicon is the preferred semiconductor material for constructing such a device, and yet silicon does not have a direct energy band gap; hence it has a very low quantum efficiency and therefore its efficiency as an electro-optical light emitter is poor. The available light emitted from such a device via recombination of carriers is generally insufficient for many imaging applications.